The present invention relates to devices for automatically operating an electrical switching mechanism in response to sensed changes in temperature. In particular, the invention relates to such devices having a sensing element positioned remotely from the electrical switching mechanism. An example would be devices having a remote sensing element for sensing freezing conditions in a device such as a refrigerator or freezer, with switching mechanism located exteriorly of the freezing compartment. Such thermostats are known to have provided, on the switching mechanism, an adjustment for selecting the desired temperature of the freezing compartment at which the electrical switching mechanism is desired to complete an electrical circuit or alternatively, have the electrical switching mechanism break an electrical circuit, in response to a predetermined sensed temperature in the freezing compartment.
In the design of thermostats employed for cold control, it is known to provide a range adjustment on the switching mechanism for enabling the operator of the refrigeration device to select a predetermined temperature about which it is desired to regulate the operation of the device. In known thermostats, the operator adjusts an input control, as for example, a rotary positionable knob, for dialing-in the selected temperature. The knob is effective to adjust the position or changing the position of a lever mechanism responsive to the remote temperature sensor thereby controlling the point at which the switching mechanism is actuated in response to a certain movement of the lever mechanism.
In known thermostat designs, the temperature responsive element is mounted to the housing means or frame and a lever mechanism is pivoted about a fulcrum on the housing means or frame, with the initial position of the lever mechanism calibrated with respect to the at-rest position of the temperature sensing element. In such devices, it is known to have a liquid filled temperature sensing bulb remotely connected to the switching mechanism by a capillary tube which provides fluid pressure to act against a diaphragm or piston which is mounted on the frame or housing means. Calibration is accomplished by moving an adjustment for prepositioning the lever means which contacts the diaphragm on piston.
In known thermostat designs, it is necessary to adjust the initial position of the lever mechanism with respect to the actuating diaphragm or piston to calibrate the lever mechanism movement with respect to the temperature responsive actuating mechanism. It is also necessary to calibrate the lever mechanism with respect to the actuation or trip point of the switching mechanism. Providing such a dual calibration within the thermostat has provided to be extremely difficult because because movement of the lever means for calibration with respect to the temperature responsive actuating member such as the diaphragm, results in movement of the lever means with respect to the switching mechanisms. Where snap-acting switching mechanisms have been employed, it has thus been difficult to calibrate the thermostat because movement of the lever means required re-positioning of the snap-acting switch actuating member. Furthermore, it is required to provide movement of the lever means for selective of the lever means for selection of the switching actuation temperature. Thermal hysteresis of the temperature responsive element also makes such calibration difficult.
It has thus long been desired to find a way or means of providing a temperature set calibration for a thermostat in which the switch actuating lever mechanism can be initially calibrated with respect to the thermally responsive element and then separately and independently calibrated with respect to the actuation point of the switching means, with the second calibration made relative to the first calibrated position of the lever mechanism with respect to the temperature responsive element.